Engine manufacturers model their engines in great detail, always based on a vast amount of established corporate knowledge and new information from contemporary programs. Outside of the OEM, however, optimal use may be made of much smaller pools of experience and data to generate reliable representations of detailed engine geometry and performance. This chapter identifies and characterizes typical sources of information that may be used for such purposes. These can be acquired from officially published data, pass-off tests by an operator, or from measurements in a maintenance facility. Consistency of the data must also be considered. With few exceptions, the key to their successful utilization in an engine performance model lies in correcting the available values to the model conditions appropriately. Issues to be considered are departures from standard day conditions as well as humidity and condensation. Which condition to choose as the cycle reference point is especially important before selecting compressor and turbine maps and moving on to off-design conditions. We give general advice in this chapter on common pitfalls and how to fill gaps with derived or hybrid data.